Cyclotron wave harmonic generator



Jan. 14, 1964 E. A. ASH 3,118,083

CYCLOTRON WAVE HARMONIC GENERATOR Filed Aug. 21. 1961 Inventor ERIC A. ASH United States Patent QYCLQTRGN WAVE HARR KGNEC GENERATOR Eric AlbertAsh, London, England, assignor to International Standard ElectricCorporation, New York,

NFL, a corporation of Delaware Filed Aug. 21, 1961, Ser. No. 132,638Claims priority, application Great Britain Aug. 30, 1969 6 Claims. or.sis-5.43

The present invention relates to electron beam tubes and moreparticularly to an electron beam harmonic wave genorator.

It is known that in any electron beam oscillator in which, before beingcollected, the beam issues from its region of interaction withelectromagnetic fields, the issuing beam will contain radio frequencycurrent components w nob are harmonics of the fundamental frequency.This is inevitable, as the amplitude of the oscillations must be limitedby some non-linear efiect.

T ere have been attempts to produce harmonic waves using klystronfrequency multipliers, but in these it is necessary to provide resonantcavities tuned both to the fundamental frequency and to the harmonicfrequency. Other arrangements eliminating the need for mechanical tuningat the fundamental frequency have been proposed using backward waveoscillations at the fundamental frequency; here however some form ofslow wave structure has hitherto been needed. At the shorter, e.g.millimetric, wavelengths it becomes increasingly difiicult to make slowwave structures. More recently, it has been shown that an electron beamwhich itself has a spatially periodic structure-e.g. the beam follows ahelical path about a given axis-wan interact with electromagnetic fieldsin a hollow waveguide which supports only waves having a phase velocitygreater than the velocity of light and which is unloaded by a slow wavestructure. An oscillation generator utilizing what is called cyclotronresonance in a hollow waveguide has been described by R. Pantell in aletter entitled Backward wave oscillations in an unloaded waveguide,published in The Proceedings of the l.R. E., vol. 47, No. 6, lune 1959,page 1146. The cyclotron reso nance phnomena are dependent on thestrength of an axial magnetic field in which the electrons and the beamrotate; this means that a straightforward frequency-multiplier usingcyclotron resonance at the harmonic frequency would entail the use of amagnetic field it times the strength of the magnetic field associatedwith the fundamental oscillations, where n is the order of the harmonic.

An object of the present invention is to provide an improved electronbeam harmonic wave generator.

in the present invention, 0 cillations at a fundamental frequency aregenerated by the use of cyclotron resonance in an unloaded waveguide.After issuing from the waveguide the electron beam is passed through acavity resonatc-r tuned to the desired harmonic frequency and supportinga mode having the same azimuthal field distribution as the current inthe electron beam and arranged for excitation b longitudinal currentcomponents of the beam. This resonator is, essentially, a klystronresonator provided with an interaction gap as in a lrlystron.

The invention will be further described with reference to theaccompanying drawings, in which:

KG. 1 shows schematically a harmonic generator according to theinvention using an annular electron beam; and,

FIG. 2 illustrates a modification of the arrangement of FIG. 1 for usewith a solid electron beam.

in the arrangement of FIG. 1, an annular beam electron gun 1 is providedfor projecting a beam of electrons through a length of hollow waveguide2 and a klystron type cavity resonator 3 to an electron collectorelectrode 4. The cavity resonator 3 is situated downstream of thewaveguide 2, i.e. between the waveguide and the electron collectorelectrode. The components mentioned are rep resented in FIG. 1 asenclosed in a hermetically sealed and evacuated envelope 5, although, inpractice, the envelope would in most cases be provided by the componentsthemselves, formed, for example, from a block of copper providing anintegral mechanical structure for the waveguide and cavity resonator.

The cathode, represented at 6, is shown connected to the negativeterminal of a D.C. potential source indicated by the battery symbol 7,the electron gun anode 8 and the other components shown on the drawingbeing connected to the positive terminal. At least the waveguide cavityresonator portions of the device of FIG. 1 are immersed in an axialmagnetic field indicated by the arrow B. On leaving the electron gun,the electrons rotate about the axis of the device under the influence ofthe magnetic field with an angular velocity w the cyclotron frequency,which frequency is given by the well known expression where 7; is thenumerical ratio of charge to mass of an electron and B is the fluxdensity of the magnetic field. Since the beam is hollow and eachelectron path is a helix about the axis of the device, the electronpaths are of a spatially periodic form in which interaction with the TEmode in a cylindrical guide can occur. As shown by Pantell, backwardwave oscillations are then set up inside the waveguide 2. When itemerges from the wave guide 2., the electron beam will contain a seriesof harmonic currents having an azimuthal variation corresponding to thatof the waveguide mode used, and these currents have axial componentswhich are much larger than the azimuthal componentsi.e. the beam will bebunched lon gitudinally. The resonator 5 is provided with an interactiongap in the same manner as for employment with klystron tubes, and isshown in l coupled through a tapered matching section of Waveguide 9 toan output Waveguide feeder it a hermetically sealing wave guide windowbeing indicated at 11. The longitudinal components of the radiofrequency waves carried on the electron beam will excite the resonator 3in a mode having a similar azimuthal variation of the electric vectortherein as the azimuthal variation of current on the electron beam, thebeam and wave interaction being essentially same as in a klystronresonator. This interaction is independent of the magnetic field, outthe latter is allowed to extend over the resonator towards the electroncollector electrode so as not to upset the electron optics of thesystem.

The arrangement of FIG. 2 is essentially similar to that of FIG. 1except that, here, a solid beam electron gun 1.2 is used oilset from theaxis of waveguide 2 and having its cathode shielded from the magneticfield. The beam issuing from the electron gun will then assume a helicalpath as indicated at 13.

The rotating beam sets up backward wave oscillations in the waveguideand longitudinal harmonic current components of the waves of the beamwill be present in the issuing beam as in FIG. 1, and are utilized inthe cavity resonator 3 in the same manner.

In the above described embodiments a circular wave guide is used and isexcited at the fundamental frequency in the TE mode. It is possible toexcite the circular guide in the TE mode at one half the cyclotronfrequency and, in this case, a square waveguide for excitation in theessentially similar TE mode of a rectangular guide can be used insteadof a circular guide.

It is to be understood that the foregoing description of specificexamples of this invention is not to be considered as a limitation onits scope.

What We claim is:

l. Ari electron beam harmonic generator comprising a hollow Waveguide,means for projecting an electron beam in said hollow waveguide, meansfor exciting fast mode backward wave oscillations at the cyclotronfrequency of said electrons in said hollow waveguide along the axis ofwhich the electron beam is projected, and a cavity resonator tuned to aharmonic of the cyclotron frequency and supporting a mode having thesame azimuthal field distribution as the radio frequency current in theelectron beam, said resonator being arranged downstream of the Waveguidefor excitation by longitudinal components of said current imposed on thebeam by the said oscillations.

2. An electron beam harmonic generator including a length of hollowcylindrical Waveguide, means for provid ing an axial static magneticfield along the waveguide, electrode means at one end of said waveguidefor projecting an electron beam along the waveguide along a helical pathin such manner that backward Wave oscillation of the cyclotron frequencyof the electrons are set up in the waveguide and on the beam, and acavity resonator surrounding the path of the beam at the other end ofsaid waveguide, said cavity being tuned to a harmonic of the cyclotronfrequency and supporting a mode having the same azimuthal fielddistribution as the radio frequency current in the electron beam, saidresonator being arranged for excitation by longitudinal currentcomponents of the electron beam.

3. An electron beam harmonic generator tube including a length of hollowwaveguide, electrode means for projecting an electron beam along thewaveguide in a helical path to excite backward mode cyclotronoscillations in the Waveguide, a cavity resonator aositioned at andspaced from the end of said waveguide opposite said electrode means forexcitation by longitud nal current components of the beam, said cavityresonator being tuned to a har- 5 monic of the cyclotron frequency andsupporting a mode having the same azimuthal field distribution as theradio frequency current in the electron beam.

4. An electron beam harmonic generator tube according to claim 3, inwhich the length of the hollow wave- 10 guide is of square section, thearrangement being such as to excite the waveguide Th mode at one halfthe cyclotron frequency.

5. An electron beam harmonic generator according to claim 1 wherein saidbeam projecting means is offset from 15 said axis and provides a solidelectron beam and includes magnetic shielding means.

6. An electron beam harmonic generator according to claim 1 including acollector electrode positioned adjacent said cavity resonator and outputcoupling means connected 20 to said resonator.

References Cited in the lie of this patent FOREIGN PATENTS France Nov.21, 1951 OTHER REFERENCES Backward Wave Oscillations in an UnleadedWaveguide; Article by R. H. Pantell, page 114-6, Proc. of 39 I.R.E., forJune 1959.

1. AN ELECTRON BEAM HARMONIC GENERATOR COMPRISING A HOLLOW WAVEGUIDE,MEANS FOR PROJECTING AN ELECTRON BEAM IN SAID HOLLOW WAVEGUIDE, MEANSFOR EXCITING FAST MODE BACKWARD WAVE OSCILLATIONS AT THE CYCLOTRONFREQUENCY OF SAID ELECTRONS IN SAID HOLLOW WAVEGUIDE ALONG THE AXIS OFWHICH THE ELECTRON BEAM IS PROJECTED, AND A CAVITY RESONATOR TUNED TO AHARMONIC OF THE CYCLOTRON FREQUENCY AND SUPPORTING A MODE HAVING THESAME AZIMUTHAL FIELD DISTRIBUTION AS THE RADIO FREQUENCY CURRENT IN THEELECTRON BEAM, SAID RESONATOR BEING ARRANGED DOWNSTREAM OF THE WAVEGUIDEFOR EXCITATION BY LONGITUDINAL COMPONENTS OF SAID CURRENT IMPOSED ON THEBEAM BY THE SAID OSCILLATIONS.